This invention relates to a thermal transfer recording system. More specifically, this invention relates to a thermal transfer recording system which utilizes a donor sheet in which the transfer layer contains an amorphous, or non-crystalline, dye phase.
A number of different printing systems utilize thermal transfer of a dye from a donor sheet to a receiving sheet. A wide variety of specific mechanisms for thermal transfer have been used, but as described in xe2x80x9cA New Thermal Transfer Ink Sheet for Continuous-Tone Full Color Printerxe2x80x9d, by M. Kutami et al., J. Imaging Sci., 1990, 16, 70-74, all fall into two broad categories. In the first of these, dyes thermally diffuse from a polymeric binder on the donor sheet into a second polymeric layer on the receiving sheet, in a process called xe2x80x9cdye diffusion thermal transferxe2x80x9d (or D2T2; this process is also sometimes called xe2x80x9cdye sublimation transferxe2x80x9d). In this type, only the dye is transferred, not the binder in which it is dispersed on the donor sheet. In the second category, commonly referred to as thermal mass transfer, or TMT, both a dye and a vehicle are transferred together from the donor sheet to the receiving sheet.
Thermal mass transfer methods may be further subdivided into those involving differential adhesion, in which a heated plug of material from the donor sheet adheres to the surface of the receiving sheet, and those involving flow penetration, in which the colorant layer on the donor sheet melts and is transferred into pores on the receiving sheet. Thermal mass transfer generally requires significantly less energy than dye diffusion thermal transfer, and, among the various thermal mass transfer processes, flow penetration may require somewhat less energy than differential adhesion. Thus, thermal mass transfer with flow penetration is the typically preferred technique for use in situations in which energy consumption must be kept low, for example, in battery-powered printing devices.
Prior art methods for thermal mass transfer with flow penetration have a number of disadvantages. Because the layers transferred tend to be relatively thick (typically in the range of 1.5-2.5 xcexcm), the pore diameter in the receiving sheet is typically required to be in the range of about 1 to 10 xcexcm, as described for example in U.S. Pat. Nos. 5,521,626 and 5,897,254. Consequently, the receiving sheet tends to scatter visible light and have a matte appearance. If it is desired to use such a method to produce a glossy image (for example, for reproduction of a photograph, where most consumers expect a glossy image), a fourth clear panel (in addition to the three primary color images) typically must be thermally transferred over the entire area of the image. Transfer of the fourth panel increases both the energy required and the time required to make a print, since four passes rather than three are required to form a fill color image. Further, if a microporous receiving sheet is not used, the durability of the image can be poor.
Such requirements for the prior art methods for thermal transfer into porous receiver materials could be alleviated by a method for transferring a molten imaging ink into receiver sheet pores substantially smaller in diameter than wavelengths of visible light. Glossy receiving sheets having such pore sizes are readily available commercially and are indeed commonly used for ink jet printing.
As the state of the art advances and efforts are made to provide new thermal transfer recording systems which can meet new performance requirements and to reduce or eliminate some of the aforementioned undesirable characteristics or requirements of the known systems it would be advantageous to have a thermal transfer recording system which can utilize relatively thin donor layers and which can utilize receiver sheets which have a relatively smooth, glossy surface. It is therefore an object of this invention to provide a novel thermal transfer recording system. It is another object of the invention to provide a thermal transfer donor material which has a relatively thin layer of transfer material. A further object of the invention is to provide a thermal transfer recording method which can utilize a relatively smooth, glossy receiver material. Still another object of the invention is to provide a donor sheet transfer material coating capable of being transferred into pores having average diameters less than about 0.2 xcexcm.
These and other objects and advantages are accomplished in accordance with the invention by providing a novel thermal transfer recording system wherein a donor element comprising a substrate bearing a layer of a thermal transfer material is heated to transfer portions of the transfer material imagewise to a receiver element. According to the invention the layer of thermal transfer material comprises a dye-containing, amorphous, (non-crystalline) phase which includes at least one dye, wherein the dye or dyes present in the amorphous phase form a continuous film. The thermal transfer material layer is not appreciably tacky at room temperature.
Optionally, and preferably, the thermal transfer material layer includes at least one thermal solvent. As will be described in detail below herein, at least a part of the thermal solvent material is incorporated into the dye-containing phase and another part of the thermal solvent material forms a second crystalline phase separate from the dye-containing phase. The crystalline thermal solvent in the thermal transfer material layer melts and dissolves or liquefies the dye-containing phase thereby permitting dissolution or liquefaction of the dye-containing phase to occur at a temperature lower than that at which such dissolution or liquefaction occurs in the absence of the crystalline thermal solvent.
There is also provided according to the invention a novel donor element for use in thermal transfer recording, the donor element comprising a substrate, or carrier layer, and a thermal transfer material layer as described above.
This invention also provides a meltable composition comprising a mixture of a first dye-containing phase as described above and a second phase comprising at least one crystalline thermal solvent.